1. Field of the Invention
The present invention generally relates to optical and electronic devices including nanocrystals, such as quantum dots.
2. Description of Related Art
Many systems currently used for short-wavelength infrared (SWIR) photodetection and imaging are achieved through epitaxial growth of compound semiconductors such as InGaAs, or chemical bath growth of polycrystalline PbS or PbSe. These techniques can result in exceptionally sensitive detectors—normalized detectivity, D*, as high as 8×1010 Jones from PbS at room temperature for example—but their deposition is generally incompatible with established silicon integrated circuit fabrication techniques. In such systems a silicon electronic read-out array and an infrared-sensitive photodetector array are fabricated separately. This non-monolithic process then necessitates a complex assembly procedure, resulting in low yield, poor resolution (e.g., at least 10× lower pixel count than a low-cost commercial silicon camera), and high cost (e.g., at least 100× greater than a silicon camera).
SWIR photodetection and imaging may also be achieved using quantum dots as a photosensitive material; however, imaging systems using quantum dots typically have relatively low gains and sensitivities. Some examples of imaging systems that utilize quantum dots, and applications thereof, may be found in the incorporated references given below.
A schematic of a ligand-capped QD nanocrystal is illustrated in FIG. 1. The QD includes a core 100, which includes a highly crystalline semiconductor region of relatively small size, e.g., from about 1-10 nm, for example about 5 nm as shown in the figure. The core is typically highly or may even be perfectly crystalline, is known to have a substantially homogeneous structure and composition. The QD is surrounded by a plurality of ligands 120 attached to its outer surface. Specifically, each ligand 120 includes a long chain, represented by the jagged line, and an end functional group 150, represented by the triangle, which connects the ligand to the outer surface of the QD.
The fabrication in solution of QDs, stabilized using suitable ligands, and typical QD characteristics such as size-tunable absorbance and emission are known. Solution-fabricated QDs may be referred to as “colloidal,” as compared with epitaxially-grown (e.g., Stranski-Krastanov-mode grown) or otherwise deposited QDs. Further details may be found in the incorporated references included below.